A static magnetic field is used by MRI scanners to align the nuclear spins of atoms as part of the procedure for producing images within the body of a patient. This static magnetic field is referred to as the B0 field. It is commonly known that increasing the strength of the B0 field used for performing an MRI scan offers the opportunity of increasing the spatial resolution and contrast resolution of the diagnostics images. This increase in resolution and contrast benefits physicians using an MRI image to diagnose a patient.
During an MRI scan, Radio Frequency (RF) pulses generated by a transmitter coil cause perturbations to the local magnetic field, and RF signals emitted by the nuclear spins are detected by a receiver coil. These RF signals are used to construct the MRI images. These coils can also be referred to as antennas. Further, the transmitter and receiver coils can also be integrated into a single transceiver coil that performs both functions. It is understood that the use of the term transceiver coil also refers to systems where separate transmitter and receiver coils are used.
However, a technical challenge in increasing the magnetic field strength used for clinical MRI is imposed by the dielectric properties of the body. The perturbations caused by the transceiver coil are used for manipulating the orientation of nuclear spins aligned with the B0 field. The problem is that dielectric properties of the body cause the wavelength λ of the transmitted RF field (so-called B1 field) to become shorter, λ˜1/B0. As the B0 field increases, the wavelength of the RF field necessary to manipulate the nuclear spin decreases. Outside of the body, the wavelength of the RF field can be on the order of meters. Within the body it is much shorter. If the B0 field is strong enough, then the wavelength of the RF pulse within the body will have decreased to the point where there are RF standing waves within the body. This causes the local perturbations to the magnetic field induced by the transmitter coil to become non-uniform, resulting in non-uniform excitation, signal strength, and contrast in standard MRI sequences. This is known as B1 inhomogeneity. This can lead to errors in contrast of diagnostic images and to the possibility of a misdiagnosis.
To counteract this effect, phased array transceiver coils are used. These coils comprise multiple elements wherein the amplitude and phase of the RF radiation generated by an individual element is controlled relative to other elements of the antenna. The process of choosing the proper phases and amplitudes to counteract the B1 inhomogeneity effects is known as B1-shimming. To perform a B1-shim, a map of the transmit field is constructed. This process is known as B1 mapping. B1 shimming is described in Yudong Zhu, U.S. Pat. No. 6,989,673 (Cited as Zhu). Zhu discloses a system composed of multiple transmit coils with corresponding RF pulse synthesizers and amplifiers.
Current methods of B1-shimming concentrate on homogenizing the B1 field. On average, the resulting shimmed B1 field is lower than the average unshimmed field. In order to have a B1 field that is strong enough to tip the nuclear spins to the desired flip angle, the transmit power has to be increased. This leads to higher Specific Absorption Rate (SAR) values. To solve this problem, it has been suggested to calculate or estimate the electric fields produced by the transmit elements and use this information to minimize the resulting total electric field. This is impractical in a clinical setting, because the calculation of the electric fields requires extensive off-line modeling and calculations.
Currently, B1 mapping and shimming is only done for individual volumes or individual slices being imaged during an MRI examination. A slice is a common term in the art used to refer to a thin volume that is imaged. The term volume is understood to refer to slices also. The B1-shimming of the individual volumes being imaged is not sufficient. For example, in a modern MRI experiment a variety of RF pulses are employed during the scan preparation phase to optimize the scan conditions such as the determination of the resonance frequency f0 or the B0-shim, each performed in separate volumes. These RF pulses are also used for magnetization preparation such as the REST, SPIR, and Inversion techniques. Such preparation MRI pulse sequences are used to manipulate the desired image contrast and image quality or to encode additional information. Further examples would be the navigator RF pulse technique that is used for motion sensing or similar RF pulses used for local Arterial Spin Labeling (ASL). Optimal image quality requires the optimal performance of these techniques.
These techniques are seriously compromised by the B1 inhomogeneities present at high B0 fields. This makes local B1 shimming mandatory. The individual pulse sequences operate on different parts of the body, and have a different local scope. Many techniques rely also on having a uniform B1 field outside of the slice or volume being imaged. For example, REgional Saturation Technique (REST) or an ASL preparation pulses are often applied outside the imaging volume of interest. For these pulse sequences, information about the B1 field is missing using standard techniques.
The static magnetic field, B0, is typically manipulated using gradient coils. The process of adjusting the B0 field using these gradient coils is referred to as B0 shimming.